In traditional networks for wireless communication, such as GSM networks, a single narrowband frequency carrier is typically used for transferring data and messages in signals between the network and a user node connected to a node of the network, typically called network node or base station, either for transmitting signals from the network node on a downlink connection to the user node or for transmitting signals from the user node on an uplink connection to the network node. Recently, increasingly advanced user terminals and devices have emerged on the market, e.g. smartphones, tablets and wireless laptops, which are suitable for services such as internet browsing and streaming of media both generating more traffic on the downlink than on the uplink. The demands for high data throughput on the downlink has therefore increased. In this description, the term “user node” is used to represent any communication equipment capable of receiving downlink radio signals from a sending network node of a wireless communications network. The user node in this context could also be referred to as a mobile terminal, mobile station, User Equipment (UE), device, etc., depending on the terminology used.
To meet the greater demand for data throughput on the downlink, the possibility of using two or more carriers in parallel on the downlink to a user node has been introduced such that the amount of data that can be communicated to the user node is basically multiplied by the number of carriers used. For example, the concept of Downlink Dual Carrier, DLDC, was specified in 3GPP GERAN, Release 7, as an enhancement to increase data throughput. The DLDC feature thus introduces two parallel carriers transmitted on two different frequencies on the downlink to the same user node, provided that the user node is capable of handling two individual receiver paths or chains for the two carriers. It may also be possible to use more carriers than two, e.g. four carriers or even more to expand the above DLDC feature. The use of multiple downlink carriers would be dependent on capabilities of the user node and it puts requirements on the user node's receiver as follows.
Since the carriers are transmitted on separate frequencies in a multi-carrier scenario for the downlink, it is necessary to use a receiver filter in the user node that encompasses all frequencies used, i.e. having a nominal bandwidth of a range from the lowest carrier frequency to the highest one used, and the receiver filter therefore typically needs to be configured with considerably wider nominal bandwidth than the filter bandwidth needed for single carrier reception. This is schematically illustrated in FIG. 1 where the filter bandwidth 102 required for reception of four carriers in parallel is much wider than the filter bandwidth 100 needed for reception of just a single carrier. As a result, a receiver with wide filter will be more sensitive to interference caused by other transmissions within the range of the wide filter bandwidth 102. Alternatively, a separate filter could be used for each individual carrier, e.g. four times the filter bandwidth 100 in the FIG. 1 example, which would more or less omit the frequencies lying between the carriers used. However, this solution would require a separate receiver equipment, also referred to as “RF front end”, for each carrier resulting in intolerable costs and space requirements for the user node.
In addition, a wide filter will let through a wider range of frequencies that lie outside its nominal filter bandwidth, as compared to a more narrow filter which is able to suppress such outside frequencies much more efficiently due to the inherent filter characteristics. This is illustrated by diagrams in FIGS. 2a and 2b showing the magnitude of passed signals over frequency through a narrow filter and through a wide filter, respectively. In these examples, it is assumed that a suppression of −60 dB is sufficient to protect the receiver from harmful interference signals. FIG. 2a illustrates the characteristics 200 of a narrow filter with a nominal bandwidth denoted NB. This filter suppresses frequencies outside its nominal bandwidth quite fast such that the range “x” of potentially harmful interference outside the nominal bandwidth NB is relatively small. In contrast, FIG. 2b illustrates the characteristics 202 of a wide filter with a nominal bandwidth denoted WB. This filter suppresses frequencies outside its nominal bandwidth considerably less such that the range “y” of potentially harmful interference outside the nominal bandwidth WB is much larger than range x.
It can thus be understood that using a wide filter for multi-carrier reception as of FIG. 2b will make the receiver far more sensitive to interference than when using a narrow filter for single carrier reception, for mainly two reasons. Firstly, the nominal bandwidth of the wide filter, indicated as the dotted area in FIG. 2b, with no suppression on neither the used carrier frequencies nor intermediate frequencies, is much wider compared to the nominal bandwidth of the narrow filter, indicated as the striped area in FIG. 2a. Secondly, it is evident from FIGS. 2a and 2b that the frequency range outside the nominal filter bandwidth where the receiver can be subjected to harmful interference, indicated as x and y respectively, is greatly increased when a wideband filter is used compared to using a narrowband filter, e.g. having a similar filter complexity. Such harmful interference may result in reduced quality and/or throughput of the downlink communication due to incorrect or failed detection and decoding of received signals.
When a wider filter is used by the user node's receiver in multi-carrier mode, the interference protection is thus greatly reduced in the receiver. In a situation with high interference from other transmissions, the receiver will have reduced sensitivity to received signals such that there is a risk of failed reception of downlink signals. In very harsh conditions the receiver may even be virtually “blocked”, i.e. it is unable to decode any downlink signals at all on the carriers used due to the interfering signals let through by the filter. In such a blocking scenario, the receiver is more or less “blind” to data reception and the additional carriers provided by multi-carrier mode will not increase the throughput compared to the single carrier mode. Consequently, the receiver will also be blind to messages and control signaling from the network that may be important for maintaining the connection and other things. Examples of such messages and control signaling include various instructions to the user node e.g. relating to power regulation, acknowledgement or non-acknowledgement of received and decoded data, request for measurement reporting, uplink scheduling, change of carriers, frequency hopping schemes, switch from multi-carrier mode to single carrier mode, and so forth.
For example, if a connection with a user node is suddenly subjected to severe downlink interference when in multi-carrier mode, the network may not be able to get across important commands to the user node for overcoming the interference, such as an instruction to switch to single carrier mode for more robust reception by using a narrow filter. As a result, the connection may be lost altogether without any possibility for remedy. Another important message to get across to the user node relates to paging of the user node for an incoming speech call or other session, which may not succeed due to interference when in multi-carrier mode such that the call is missed.